Just to clear things up a bit, I am what most would call a Halo fanboy(though, I do have experience in playing both Halo and Call of Duty). So, I request my dear opponent to refrain from labeling me as a ten year old COD crybaby.

I sincerely apologize for forfeiting the previous round as I was rather busy during that 24 hour window.

Both the Halo and COD series are first-person shooters with linear gameplay. There isn't much of a difference between the two games in terms of concept but its the pace in which the game is played in is what distinguishes the two.

Halo is a much more slower paced game compared to COD. The avatar in Halo have a "double health system" which is consisted of shield points and hit points. The shield points must be depleted before the hit points can be. This "double health system" effectively makes an avatar harder to kill, requiring up to half a clip of sustained fire from the assault rifle(MA5B-MA5D) before the said avatar is killed.

COD on the other hand, has a more realistic health system. A few shots from even a pistol to the torso of an avatar would be sufficient to kill. Grenades in this game deal a larger percentage of damage compared to the grenades(minus a stuck plasma grenade) in Halo.

Both the points above show the difference in the health and damage concepts. In Halo, avatars are harder to kill; In COD, avatars are easier to kill. This means that even "pros" in COD can be killed by "noobs". In other words, surviving is more difficult in COD whereas in Halo, a "pro" might be able to outmaneuver several "noobs" at once because he/she has more health to kill the "noobs".

Killing other avatars is definitely much easier in COD than in Halo, I concede that. But that also means that it is also a lot easier to die in COD compared to Halo. Maintaining a respectable KDR(Kill-to-Death Ratio) in COD, is indubitably more difficult to accomplish in COD, hence it takes more skill.

Like Domr said, how is this even measurable? The gameplay is practically identical, so the main discrepancies are weapons, maps, and other customizable options. And you can't really use online multiplayer as an example, as that isn't the game requiring skill - it's your opponent.